One form of clamping mechanism for the plate cylinder of a printing press is shown in German Pat. No. 2,200,187. However, a disadvantage of this mechanism is that the rear clamping rail is disposed, without any lateral alignment means in the cylinder gap, on an angle bar displaceable circumferentially in the gap, and an increased clamping travel is required to clamp it from its unclamped position, in which it is pushed against its gap wall, by means of a quick-action clamping mechanism with a lever transmission system. The clamping movement in this arrangement is in the circumferential direction of the cylinder. When the printing plate is in the released condition, this bar can be displaced in the direction of the gap wall facing the rear clamping bar and be disengaged from the gap wall by an eccentric through a lever transmission system. Since the rear clamping bar is disposed between two compression springs, any inequality in the disengaged positions of the axially distributed clamp screws (of which there are at least two) affects the parallelism of the displacement of the rear clamping bar in relation to the gap wall when the plate is clamped by means of the quick-action clamping mechanism.
If during the preparatory work for clamping a new printing plate, the clamp screws in the rear clamping bar are not released by exactly the same amounts, then during the quick-action clamping operation, the rear clamping bar, which is not guided accurately laterally, tilts, and the rear printing plate end is skewed, and in the case of a sensitive printing plate this means that the plate is elongated on one side and distorted. It is then impossible to clamp the plate in exact registration, since the plate deformation cannot be cancelled out even by a subsequent correction using the clamp screws. Nor is it possible to obtain a defined clamping force over the printing plate width if the two clamping bars are not in a parallel position, because of the above-mentioned arrangement of two counter-acting compression springs, i.e. one between the clamping bars and one between the eccentric and a rocking lever. Automatic re-clamping by way of the compression spring between the clamping bars is possible theoretically, but in practice the required clamping force cannot be so applied, particularly in the case of large printing plate formats. Another disadvantage is that the rocking lever mounting must be very stable, with the result that it requires a considerable amount of space.
In contrast to the above, the quick-action clamping mechanism disclosed in German Pat. No. 2,754,080 does not include a series arrangement of a quick-action clamping mechanism for pre-clamping and a clamping mechanism for final clamping. Although tilting of the rear clamping bar can be avoided in this way, because the quick-action clamping of the printing plate is carried out with an accurately defined force by means of a single operation through parallel guidance of the rear clamping bar in the cylinder gap by means of a symmetrical scissors lever linkage, this mechanism, as shown in FIG. 1 of the patent, does not permit lateral alignment or alignment in sections circumferentially with respect to the cylinder. Therefore, it is not possible to correct prints which have changed due to stresses in hygroscopic printing material or in the elastic rubber blankets, nor is it possible to adjust individual printing plates to one another in the case of multi-color machines. It is precisely these corrections, of course, which are fundamental importance. In addition, the quick-action and final clamping functions cannot be separated in order to avoid any undesirable effects.
German Pat. No. 3,127,586 discloses a quick-action clamping mechanism for parallel clamping, in which an angle bar for clamping printing plates of different print lengths is disposed to be adjustable and lockable in a number of fixed positions parallel to the print end, but in which only a limited clamping force can be produced during quick-action clamping by a leaf spring acting on the angle bar. The mechanism shown in FIG. 2 of this patent can be provided with a divided clamping bar but section-wise correction of the print is essentially not possible because a divided rear clamping bar is again coupled by way of the quick-acting clamping mechanism disposed symmetrically in the cylinder center plane. Thus the quick-action clamping and final clamping functions are not capable of separation, so that undesirable effects on the clamping force are inevitable.